The present invention relates to systems for controlling the operation of an electric motor, a more particularly to such systems in which unidirectional electric switches are employed to control the application of electricity to the motor.
The operation of three-phase electric motors is frequently controlled by an apparatus which regulates the flow of electricity to the motor in order to reduce power consumption during the starting of the motor and control the motor speed during normal running conditions. Such apparatus commonly use thyristors to switch each phase of electricity to the motor. A typical configuration consists of two silicon controlled rectifiers (SCR) connected in an inverse parallel relationship for each phase of the electricity. An electronic control circuit senses the polarity of the alternating voltage for each phase and triggers the corresponding SCR's which are forward biased during each half cycle of the voltage. The control circuit also regulates the phase angle of the voltage cycle at which each SCR is triggered to control the magnitude of electricity applied to the motor and thereby control the power consumption and speed. For example, during the start up of the motor, the phase angle at which each SCR is triggered is gradually decreased so as to slowly increase the amount of electricity applied to the motor.
An alternative control system replaced one of the SCR's in each inverse parallel connected pair with a diode. The diode/SCR combination improves the stability of the motor control during starting. In this case, the connection of this phase of the electricity source to the motor was conductive for the entire half cycle of the voltage during which the diode was forward biased. This alternative eliminated the need to provide a trigger signal during this half cycle of the voltage for that phase. The problem with this circuit is that each diode remains conducting even when the motor is turned off by the control circuit. Although the SCR's in the control circuit are turned off, current still can flow through the diodes and leak to the motor housing which is typically grounded. This leakage current deteriorates the insulation and components within the motor.
Previous controllers overcame this problem by utilizing two inverse parallel connected SCR's in each phase and triggering one of the SCR's during the entire half cycle of the supply voltage during which the SCR was forward biased. As a result of this triggering, one of the SCR's acted as a diode. Although this operation solved the problem of current leakage when the motor was turned off, it still required a complex control circuit to apply a second trigger pulse for each phase of the motor supply voltage and consumed additional power for the triggering.